1. Field of the Invention
The present invention relates to a current to voltage conversion circuit for a photo detector integrated circuit (PDIC) employing a gain switching circuit.
The present invention relates, more particularly, to a current to voltage conversion circuit for improving the speed of a photoelectric conversion device and frequency characteristics of an amplifier by improving a method for switching the gain of an amplification circuit for a PDIC, wherein photocurrent generated in the photoelectric conversion device, such as a photodiode, is transferred to the amplifier by means of current mirroring, thereby raising a bias voltage to the photoelectric conversion device and enhancing a response speed thereof.
The present invention further relates to a current to voltage conversion circuit for adjusting the amount of current generated in a photoelectric conversion device through control of a resistance ratio of a current mirror circuit so that a fixed feedback resistor can be used for an amplifier irrespective of modes, so as to enhance frequency characteristics of the amplifier.
2. Description of the Related Art
In general, an optical pickup apparatus, which reads or writes information from/on an optical storage medium such as a compact disc-rewritable (CD-RW), employs a PDIC for detecting light that is projected from a laser diode and then reflected from the optical storage medium, or optical disc, and converting the detected light into an electrical signal.
In a writable CD, such as the CD-RW, a write operation is performed by projecting laser beams with strong power onto the surface of the CD to form pits thereon. During this write operation of the CD, excessive power may be applied to the PDIC, resulting in a large amount of current being generated from the PDIC. Such current from the PDIC saturates an amplification circuit of a current to voltage conversion circuit for the PDIC, thereby causing a transient response characteristic of the amplification circuit to be distorted.
Two methods have been conventionally proposed to prevent the amplification circuit of the current to voltage conversion circuit from being saturated during the write operation of the CD. One is to use a limiter circuit that detects a voltage or current in the amplification circuit and supplies limiter current for compensation for feedback current if the detected voltage or current is above a predetermined value. The other is to use different amplifiers and feedback resistors with different resistances according to read and write modes to prevent the amplification circuit of the current to voltage conversion circuit for the PDIC from being saturated during the write operation.
In a different manner from a read only PDIC, a PDIC for a read/write system can be used in various application fields such as the CD-RW, a digital versatile disc-rewritable (DVD-RW), etc., so it needs a mode switching operation based on each of the application fields. Because an output voltage from the PDIC has a different level based on each mode, a gain switching function is essentially provided in the PDIC, but a high-resistance resistor is required as a feedback resistor to be connected to an amplifier in some modes. The use of this high-resistance feedback resistor not only reduces the overall response speed of the PDIC, but also deteriorates noise characteristics thereof. Therefore, there is a need to make up for such problems.
FIG. 1 is a circuit diagram showing the configuration of a conventional current to voltage conversion circuit employing a gain switching circuit. In this drawing, carriers are generated in a photodiode 11 due to light incident thereon, thereby causing current IPH to be generated in the photodiode 11. The current IPH pulls current IF from an output terminal VOUT1 or VOUT2 of the current to voltage conversion circuit, so the current IF flows through a resistor RF1 and capacitor CF1 or a resistor RF2 and capacitor CF2. At this time, a potential difference is generated across the resistor RF1 and capacitor CF1 or the resistor RF2 and capacitor CF2, thereby allowing the current IF to be converted into a voltage Vout1 or Vout2 as follows: IF×RF1=Vout1 or IF×RF2=Vout2. Because input current to an amplifier 12 is close to 0, IF≈IPH at an input terminal Tin thereof.
In a read mode where a weak optical signal is inputted to the photodiode 11, a switch SW1 is turned on in response to a switching control signal provided from an external control block (not shown), whereas a switch SW2 is turned off in response thereto. As a result, the voltage IF×RF1=Vout1 is outputted at the output terminal VOUT1 through a read amplifier 13. Alternatively, in a write mode where a strong optical signal is inputted to the photodiode 11, the switch SW1 is turned off, whereas the switch SW2 is turned on. As a result, the voltage IF×RF2=Vout2 is outputted at the output terminal VOUT2 through a write amplifier 14.
The amplifier 12 ideally has an infinite gain and constant direct current (DC) input voltage, but practically a limited gain and variable DC input voltage, resulting in an offset voltage being generated therein. For this reason, a resistor RC is connected to another input terminal of the amplifier 12 to set the offset voltage of the amplifier 12 to 0.
FIG. 2 is a circuit diagram illustrating the operation of a current mirror that is used as a constant current source in a conventional circuit constructed with transistors.
With reference to FIG. 2, if Kirchhoff's current law is applied to a voltage Vx on the assumption that the same amount of current IREF as that of current IPH generated with the input of an optical signal to a photoelectric conversion device, for example, photodiode, 21 flows in a current source 22, the result is:IC1+IB1+IB2=IREF 
If amplification factors of transistors Q1 and Q2 are β.             I      C1        +                  I        C1            β        +                  I        C2            β        =      I    REF  
Assuming that IC1≈IC2, IC2=IO, thus             I      O        +          2      β        =      I    REF  
Arranging the above equation with respect to IO, the result is:       I    O    =            β              β        +        2              ·          I      REF      
Here, assuming that β=30, IO becomes about 0.93·IREF. That is, the same amount of current IREF as that of current generated in the photoelectric conversion device 21 flows from an emitter of the transistor Q1 to a collector thereof, and almost the same amount of current IO as that of the current IREF is generated in the transistor Q2 by means of mirroring.
FIG. 3 is a detailed circuit diagram of the conventional current to voltage conversion circuit for the PDIC employing the gain switching circuit, shown in FIG. 1.
In FIG. 3, the current mirror 12 is a current source for supplying current to the read amplifier 13 and write amplifier 14, which includes a pair of differential amplifiers Q1 and Q2 and active loads Q3 and Q4. The differential amplifiers Q1 and Q2 and active loads Q3 and Q4 are of a typical bipolar junction transistor (BJT) type. The current mirror 12, read amplifier 13 and write amplifier 14 constitute one amplification circuit 15.
The read amplifier 13 is used for, for example, a digital versatile disc-read only memory (DVD-ROM) to convert current generated when a weak optical signal is inputted to the photodiode 11, into a voltage and amplify the converted voltage. The high-resistance feedback resistor RF1 is connected between the output terminal VOUT1 and the input terminal Tin.
The write amplifier 14 acts to convert excessive current generated when a strong optical signal is inputted to the photodiode 12, into a voltage and amplify the converted voltage. The relatively low-resistance feedback resistor RF2 is connected between the output terminal VOUT2 and the input terminal Tin to limit the output voltage to a constant value even though excessive current is generated.
However, the above-mentioned conventional current to voltage conversion circuit for the PDIC employing the gain switching circuit has a disadvantage in that the feedback resistors having their different resistances based on the modes are required for gain switching when current is generated according to the input of an optical signal, thereby reducing the overall response speed of the PDIC and deteriorating noise characteristics thereof.
Further, in the conventional current to voltage conversion circuit for the PDIC employing the gain switching circuit, the mode switching operation is performed based on the switching control signal provided from the external control circuit, and the current source for current supply is positioned between the photodiode and the read and write amplifiers, resulting in a long distance from the photodiode to the read and write amplifiers. As a result, noise characteristics of the PDIC are subject to deterioration.
Furthermore, in the conventional current to voltage conversion circuit for the PDIC, as shown in FIG. 3, a bias voltage is applied to the photoelectric conversion device 11 through the current mirror 12 that acts as the current source in the current to voltage conversion circuit. As a result, provided that a drive voltage VCC is 5V, a reverse bias voltage to the photoelectric conversion device 11 will be no more than 2.1V or 2.5V. Note that the photoelectric conversion device 11 has a depletion layer whose size increases with the level of a bias voltage applied thereto. In this regard, the limited bias voltage causes an insufficient extension of the depletion layer of the photoelectric conversion device 11 and, in turn, an increase in parasitic capacitance thereof. Consequently, a product of the parasitic capacitance of the photoelectric conversion device 11 and the feedback resistance Rf increases, resulting in an increase in a time constant influencing high frequency characteristics. In conclusion, the limited bias voltage adversely affects high frequency characteristics of the current to voltage conversion circuit.
On the other hand, Japanese Patent Laid-open Publication No. Heisei 11-186856 discloses a current to voltage conversion circuit for switching feedback resistors with different resistances according to read and write modes to prevent transistors in an amplification circuit from being saturated. In the disclosed circuit, however, in a similar manner to that stated previously with reference to FIG. 3, the feedback resistances reduce the overall response speed of a PDIC and deteriorate noise characteristics thereof.